1. Field
The present embodiment relates to a tandem thin-film silicon solar cell and a method for manufacturing the same.
2. Description of the Related Art
Recently, energy has become the most serious problem that impacts on human's future life due to the high oil price and the global warming caused by an excessive emission of CO2. Many technologies for renewable energy have been developed, such as wind power generation, bio-energy, hydrogen fuel cells and the like. However, solar energy is almost infinite and clean energy as the origin of all energies, and hence solar cells using sunlight have become into the spotlight.
The power of sunlight incident on the surface of the earth is 120,000 TW. Theoretically, if only 0.16% of the surface area of land on the earth is covered with solar cells having a conversion efficiency of 10%, the solar cell can generate power of 20 TW per year, which is two times greater than global energy consumption.
Practically, the global solar cell market was explosively developed with all annuall growth rate of 40% during the past 10 years. Currently, 90% of the solar cell market is shared by bulk silicon solar cells such as single-crystalline silicon solar cells or multi-crystalline silicon solar cells. However, production of solar-grade silicon wafer which is a main source does not catch up with explosive demand, and the solar-grade silicon wafer is running short all over the world. Therefore, the solar-grade silicon wafer is a crucial factor in lowering production cost.
On the other hand, thin-film silicon solar cells using a absorber based on hydrogenated amorphous silicon (a-Si:H) can reduce the thickness of silicon to less than 1/100 of that of silicon used in the bulk silicon solar cells. Further, it is possible to produce large-size and low-cost thin-film silicon solar cells.
However, the so-called Staebler-Wronski effect is blocking the commercialization of the thin-film silicon solar cells. The Staebler-Wronski effect refers to degradation caused by the photocreation of dangling bond accompanied by non-radiative recombination of electron-hole pairs photogenerated in an absorber based on amorphous silicon when light is irradiated onto a thin-film silicon solar cell.
A large number of studies were conducted during the past thirty years to reduce light-induced degradation of an amorphous silicon-based intrinsic abosorber. As a result, it has been found that kinds of two edge materials at a phase boundary between amorphous silicon and microcrystalline silicon are good intrinsic absorbers with a low light-induced degradation ratio. One is hydrogenated intrinsic protocrystalline silicon (i-pc-Si:H) which exists just before the amorphous silicon to microcrystalline silicon transition. The other is hydrogenated intrinsic microcrystalline silicon (i-μc-Si:H) having a crystal volume fraction of 30 to 50%.
Meanwhile, although the degradation against light soaking is reduced, there is a limit to performance achieved by single-junction thin-film silicon solar cells. For this reason, to obtain high stabilized efficiency, there have been developed a double-junction thin-film silicon solar cell in which a top cell based on amorphous silicon is stacked on a bottom cell based on microcrystalline silicon and a triple-junction thin-film silicon solar cell more developed from the double-junction thin-film silicon solar cell.
The double-junction or triple-junction thin-film silicon solar cell is referred to as a tandem solar cell. The open-circuit voltage of the tandem solar cell is the sum of voltages of respective unit cells, and the short-circuit current of the tandem solar cell is the minimum current among short-circuit currents of the respective unit cells.
In a tandem solar cell which comprises a top cell and a bottom cell forming heterojunction, the optical band gap of an intrinsic absorber is gradually decreased as light is incident from the top cell to the bottom cell. Accordingly, light with a wide range of spectrum is absorbed into the respective unit cell by separating light, so that quantum efficiency can be enhanced.
Further, the thickness of an intrinsic absorber of the top cell based on amorphous silicon having relatively serious degradation caused by light soaking can be decreased, so that a degradation ratio can be reduced, and accordingly, a highly stabilized efficiency can be obtained.
The stability of the tandem solar cell against light soaking is influenced by the stability of an intrinsic absorber of each of the unit cells against light soaking and the thickness of an intrinsic absorber of the top cell sensitive to light soaking.
Therefore, an intermediate reflection layer capable of strengthening internal reflection is interposed between a top cell and a bottom cell in a double-junction thin-film silicon solar cell or between a top cell and a middle cell in a triple-junction thin-film silicon solar cell, so that the thickness of a absorber based on hydrogenated intrinsic amorphous silicon is decreased in the top cell sensitive to light soaking, and desired short-circuit current is maintained and improved. Accordingly, a degradation ratio can be reduced, and a highly stabilized efficiency can be obtained.
At this time, the intermediate reflection layer of the tandem solar cell requires a transparent material which has a small light absorption, a high electric conductivity and a large difference of refractive indices between the intermediate reflection layer and a silicon thin-film.
Since the refractive index of the silicon thin-film is 3.5 to 4.0, a material having a refractive index smaller than that of the silicon thin-film has been developed as the intermediate reflection layer. The material is a zinc oxide (ZnO) thin-film having a refractive index of around 1.9 (S. Y. Myong et al., Applied Physics Letters, 2007, Vol. 90, p. 3026-3028, Y. Akano et al., EP 1650814A1, Y. Akano et al., EP1650813A1) or a hydrogenated n-type mixed-phase silicon oxide (n-SiOx:H) thin-film having a refractive index of around 2.0 (C. Das et al., Applied Physics Letters, 2008, Vol. 92, p. 053509, P. Buehlmann et al., Applied Physics Letters, 2007, Vol. 91, p. 143505).
Here, the mixed phase refers to a structure in which crystalline silicon grains are incorporated into the tissue of a hydrogenated amorphous silicon oxide (a-SiOx:H), and is frequently referred to as nanocrystalline or microcrystalline.
On the Ramam spectrum of a thin-film, a transverse optic (TO) mode crystalline silicon peak exists near 520 nm.
The zinc oxide intermediate reflection layer has excellent transmittance and vertical electric conductivity, so that the efficiency of the tandem solar cell can be improved. However, in mass production of large-area solar cells, problems such as shunts are generated in zinc oxide when the zinc oxide intermediate reflection layer is patterned through laser scribing.
Since the hydrogenated n-type mixed-phase silicon oxide thin-film is a kind of silicon alloy, laser patterning can be simultaneously performed with respect to top and bottom cells using the same laser wavelength. Accordingly, the mass production yield of the solar cell can be increased, and the layout of mass production lines can be simplified. As the content of oxygen is increased, the refractive index is decreased, and thus the internal reflection is increased. However, the crystal volume fraction is decreased, and the electric conductivity is lowered. Therefore, the series resistance is increased, and the fill factor (FF) is decreased.